Information storage devices are approaching their physical capacities to store information serially, i.e., as a long continuous stream of 1's and 0's. In order for storage devices to increase the amount of stored information which can be readily accessible, present technologies are beginning to shift away from semiconductor memories, magnetic recording media, and tape storage devices due to inherent limitations such as slow access times and limited capacities and into multi-dimensional storage media which enable very high capacities with fast parallel access.
One such multi-dimensional device is the holographic storage medium wherein entire pages of data are intermingled with each other such that numerous pages of data are recorded in a common volume of the storage media. Such volumetric storage can vastly increase the amounts and kinds of information which can be stored. These systems have the primary advantage of retrieving a page of data in a single read and storing an entire page in a single write operation thus increasing data access speeds.
In holographic storage, in general, two-dimensional data pages are optically recorded via a complex interference process wherein the data pages are initially stored in digital form on a page composer or spatial light modulator. The presence or absence of light at a given bit location represents either a 1 or 0 respectively. The data pages are subsequently retrieved by illumination of the hologram by a matched two-dimensional array of photo-detectors.
More specifically, holography is a form of 3-dimensional recording utilizing coherent light with photo-sensitive media to record and reproduce the amplitude and phase of an optical wavefront. In part, preparation of the hologram involves the channeling of a highly coherent light source, such as a laser, to illuminate both a subject and a photo-sensitive medium wherein an interference pattern is generated between the beams of light scattered from the subject (referred to as the object wavefront) and the light impinging directly on the medium (referred to as the reference wavefront). A lens is interposed between the input data field and the recording medium such that a Fourier transform of the input data is formed on the recording medium. This Fourier transform represents the spatial frequency content of the input data. It is this light distribution that interferes with the reference beam to form the hologram. A recording medium having sufficient resolution and dimension is positioned in the region of overlap to record a sample of the interference pattern. The interference pattern, which occurs in the region of overlap of the two wavefronts, typically consists of a plurality of bright regions wherein the object and reference wavefronts constructively interfere with one another and thus enhance one another, and a plurality of dark regions wherein the wavefronts destructively interfere and thus cancel each other out. In addition to recording the interference between the object and reference wavefronts, the hologram also records the interference between the many wavefronts coming from the object (data patterns) by itself, i.e., each 1 in the data field generates a wavefront that interferes with the wavefronts from the other 1's in addition to interfering with the reference wavefront. The object wavefront can be reconstructed via diffraction from the recording, now a hologram, by illumination with a replica of the original reference wavefront at the same angle of incidence and wavelength as was used during the recording process. The reconstructed object wavefront contains all the amplitude and phase information of the original object wavefront and can be processed as if the original information was still in place.
Because the hologram may have a relatively large thickness of up to several millimeters, each of these individual wavefronts interact with the many secondary holograms to also diffract light into the image. Although the diffraction process from secondary holograms is much weaker than diffraction from the primary reference-beam/object-beam hologram, (because the beam intensities forming the secondary holograms are much weaker and the beams illuminating these holograms are also weaker), broad areas of 1's in the data field should be avoided because these beams of diffracted light will appear to the sides of the desired beam and may cause light to fall on a dark region ( or 0) thereby changing that data bit. The concentration of light into localized regions would cause a situation similar to over-exposure in photography, (i.e., the dynamic range of the recording medium may be exceeded) and the recording and reconstruction would not be a true reproduction of the original data.
An example of such a detrimental code pattern is illustrated in FIG. 1A wherein a large block of light (1's) is recorded adjacent to a similarly large block of dark (0's). Although this pattern consists of 50% 1's and 50% 0's, relatively large regions of 1's may expose adjacent regions of 0's. In order to prevent the concentration of light into localized regions, it is important that the recorded information be spread evenly over the available spatial range.
Even distribution of the bits of 1's and 0's across the data page requires more consideration than merely the avoidance of light regions juxtaposed to dark regions. The property of periodicity affects the Fourier transform of the input data that is formed on the recording medium. This Fourier transform represents the spatial frequency content of the input data and it is this light distribution that interferes with the reference beam to form the hologram. For instance, FIG. 1B represents another detrimental data pattern even though that data page also has 50% 1's and 50% 0's. This highly periodic pattern has distinct peaks of intensity in the Fourier transform plane due to the periodicity of the data patterns. Therefore, in order not to permit variations in the intensity of the reconstructed image and so as not to permit wide variations in the ratio between the signal beam and the reference beam during recording, the 1's and 0's in any region of the data page should be balanced and evenly distributed at the same time.